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Date: Fri, 12 Feb 2016 21:40:27 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <Waiman.Long@hpe.com>
Cc: Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ding Tianhong <dingtianhong@huawei.com>,
        Jason Low <jason.low2@hpe.com>, Davidlohr Bueso <dave@stgolabs.net>,
        "Paul E. McKenney" <paulmck@us.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <Will.Deacon@arm.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Waiman Long <Waiman.Long@hp.com>
Subject: Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to
 mutex_optimistic_spin()
Message-ID: <20160212204027.GZ6357@twins.programming.kicks-ass.net>
References: <1455298335-53229-1-git-send-email-Waiman.Long@hpe.com>
 <1455298335-53229-2-git-send-email-Waiman.Long@hpe.com>
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On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote:
> @@ -358,8 +373,8 @@ static bool mutex_optimistic_spin(struct mutex *lock,
>  			}
>  
>  			mutex_set_owner(lock);
> -			osq_unlock(&lock->osq);
> -			return true;
> +			acquired = true;
> +			break;
>  		}
>  
>  		/*
> @@ -380,7 +395,10 @@ static bool mutex_optimistic_spin(struct mutex *lock,
>  		cpu_relax_lowlatency();
>  	}
>  
> -	osq_unlock(&lock->osq);
> +	if (!waiter)
> +		osq_unlock(&lock->osq);
> +	if (acquired || waiter)
> +		return acquired;
>  done:
>  	/*
>  	 * If we fell out of the spin path because of need_resched(),

Is there a reason to not also preempt in the wait-loop? Surely the same
reason is still valid there too?